The increasing use of Carbon Fiber Polymer Composite (CFPC) as a lightweight material in automotive and aerospace industries requires more effective and reliable joining techniques. The surface treatment is a critical step governing the quality of adhesive bonded joints. Traditional surface preparation methods of composite surfaces leave a resin-rich surface layer on the composite, which is susceptible to the nucleation of cracks that may limit its structural integrity. The CFPC surface also contains chemicals, such as mold releases and contaminants, which often require removal prior to adhesive bonding. Current state-of-the art surface preparation for composites is typically accomplished through multiple mechanical/chemical processes, which lack the speed and repeatability needed for high-throughput manufacturing. They also involve chemicals that often require special handling. Many adhesive suppliers formulate adhesives to accommodate less than ideal surfaces, at the expense of structural performance.
Aluminum (Al) surfaces contain oxides and lubricant oils that are detrimental to the adhesive joining. Surface treatments aim at modifying the Al surface to attain contaminant removal, wettability with either primer or adhesive, and highly roughened surfaces. Traditional surface preparation techniques for Al used in the industry for bonding aluminum include grit blasting, solvent wiping followed by abrading (with a ScotchBrite® pad), or anodization. The first two are more common, whereas anodization is more commonly seen in aerospace applications where this more expensive and rigorous preparation is necessary to meet stringent specifications. Due to the inherent variability in grit blasting or abrasion techniques, a more controlled process would be desired.
Surface preparation is one of the main challenges for bonding CFPC with Al, both in consistent quality and productivity. The major roadblocks in joining CFPC and Al is related to the inherent poor mechanical strength of the resin, as the adhesive does not directly contact the CFPC but the resin rich layer on the CFPC surface. Moreover, the Al surface may be too smooth to allow an excellent mechanical bonding with the adhesive. Additionally, the surfaces of both materials as noted contain contaminates, which are residual from their forming/molding operations. Aluminum surfaces contain oxides and lubricant oils. Composites contain mold releases. Traditional preparation of CFPC surfaces often include chemical removal of mold releases and other handling aids but, even under the most ideal scenarios, still leaves a resin rich surface layer on the composite which is susceptible to Mode I fracture sensitivity.
To date, joining CFPCs and metal components, made of aluminum 5000, 6000, or 7000 series, titanium, magnesium, and steels is made by simply overwrapping the CFPC composite over the aluminum or using specially formulated adhesives coupled with extensive surface preparation techniques. These processes are empirical, employing several steps, such as labor-intensive surface preparation methods that are incompatible with the degree of automation required in automotive applications. In addition to the cost and floor space requirements, manual surface preparation introduces a significant variability in the overall joint integrity. The technique is not limited only to aluminum but to any metal, although Al and CFPC materials are common in the automotive and aerospace industries.
The use of one-beam laser for serial structuring, which can produce one geometrical feature per laser spot or line raster, has been used in last two decades for CFPC. Niino et al. [H. Niino, M. Nakano, S. Nagano, H. Nitta, K. Yano, A. Yabe, Excimer Laser Ablation of Polymers and Carbon Fiber Composites, Journal of Photo polymer Science and Technology, Vol. 3 (1990), pp. 53-56.] used a 308 nm XeCl excimer laser to induce surface micromodifications onto polymers and carbon fiber composites. The use of an excimer laser treatment, selective removal of the organic matrix without any degradation of fiber reinforcements, was investigated by Galantucci et al. [Galantucci, L M; Gravina, A; Chita, G; Cinquepalmi, M, Surface treatment for adhesive-bonded joints by excimer laser, Composites Part A-Appl. Sci. and Manuf., Vol. 27, pp. 1041-1049, 1996.] and Benard et al. [Benard, Q., Foisa, M., Grisel, M., Laurens, P., Surface treatment of carbon/epoxy and glass/epoxy composites with an excimer laser beam, Int. J. of Adhesion and Adhesives, Vol. 26, pp. 543-549, 2006] [Benard, Q.; Fois, M.; Grisel, M.; Laurens, P.; Joubert, F., Influence of the Polymer Surface Layer on the Adhesion of Polymer Matrix Composites, J. of Thermoplastic Comp. Mat., Vol. 22, Pages: 51-61, 2009]. Warren et al. [Warren, C. D., Paulauskas, F. L., and Bowman, R. G., “Laser Ablation Assisted Adhesive Bonding of Automotive Structural Composites,” ORNL/CP-102637, ICCM-12, Paris France, Jul. 3-9, 1999] also used a KrF excimer laser to process glass fiber-reinforced polymer matrix composites. Significant glass fiber damage was found due to the ablation process, as the glass fibers acted as a lens focusing the laser energy to the backside of the fibers and thus produced microgrooving of the fiber along the side of the fiber farthest away from the incident direction. Lima et al. [Lima, M. S. F.; Sakamoto, J. M. S.; Simoes, J. G. A.; Riva, R., Laser processing of carbon fiber reinforced polymer composite for optical fiber guidelines, Lasers In Manufacturing (LIM 2013), Physics Procedia, Ed. by Emmelmann C; Zaeh M F; Graf T; Schmidt M, Vol. 41, pp. 565-573, 2013] used a 20 W of a Nd:YAG pulsed laser to producing fiber optical guidelines in carbon fiber reinforced polymer (CFRP) composites using laser texturing and machining. The size of laser-created surface patterns is identical to the laser spot size. Detrimental microcracks can be propagated into the subsurface level.